This week’s news in the Astronomical Journal that Mike Brown and Konstantin Batygin of CalTech have predicted a ninth planet got the usual barrage of silly media headlines about ‘discovery’.
The thing is, Brown and Batygin haven’t found anything. They’ve hypothesised that there’s a planet. And they are not the first to wonder about some kind of outer-outer planet – there’s been speculation for decades that an Earth-plus size world might be lurking in the dim reaches of the outer solar system. The problem is that it wasn’t found last time gravity-pertubation calculations were made. Or the time before that. Or the time before the time before that. Or… yah, you get the picture.
What makes this hunt different, though, is that Brown is a planet-finder of long experience – among other discoveries, he found Eris (nee Xena) which triggered the IAU’s 2006 re-think about what defines ‘planet’. So what’s their hypothesis? By analysing the orbits of six Kuiper Belt objects – including the dwarf planet Sedna – which probably wouldn’t be where they are if something hadn’t shepherded them, Brown and Batygin postulate a planet, between four and ten times the mass of Earth in an inclined and egg-shaped orbit far beyond Pluto and the Kuiper Belt. To put that in perspective, the upper mass limit is almost as much as Uranus.
The principle is sound for a variety of reasons. That ‘discovery by calculation’ is how Neptune was found. In 1821, Alexis Bouvard published data showing where Uranus should be – but the planet failed to co-operate. The wobble in Uranus’ orbit was put down to the tug of an unknown planet. British mathematician John Couch Adams got to work in the early 1840s figuring out where that unknown planet might be. So did French mathematician Urbain le Verrier. The first search at the urging of Astronomer Royal Sir George Airy, in June 1846, failed to find anything. But then in September Johann Galle of the Berlin Observatory found the new world where le Verrier had predicted.
For all that, the perturbation discovery method doesn’t always work. One of the reasons is that the so-called ‘n-body’ equation can’t be solved. The maths only work for two bodies, not more (mathematically, n = 2 is fine, but n = >2 isn’t). That means the answers have to be modelled (approximated). And sometimes, planets move oddly for other reasons. In the nineteenth century there was speculation that specific shifts (precession) in Mercury’s perehelion were caused by the tug of an unknown world orbiting closer to the Sun and invisible to us against the Sun’s glare. That was disproven by Albert Einstein, whose theory of General Relativity accounted for Mercury’s precession without recourse to another planet. Mercury was exactly where Einstein said it should be, which was one of the first proofs of his theory.
In the outer solar system, pertubation analysis still applies – though as I say, prior studies haven’t added up to a discovery. The current one hasn’t, yet, either – though the search hasn’t got going in earnest. On the plus side, Brown and Batygin, with colleague Hayden Betts, earlier modelled how the main planets ended up where they are. By that 2012 hypothesis, to get the current setup out of the chaotic bash-or-flick pool game of the solar system’s formation, a fifth giant planet about the mass of Uranus (or a little less) was flung either out of the solar system altogether, or into a distant orbit.
If anything’s orbiting where Brown and Batygin propose, it’ll be incredibly dim despite its size – which explains why it hasn’t been found by accident so far. If it turns up, thanks to the criteria now used to define ‘planet’ – which knocked Pluto off its perch in 2006 – it would be Planet Nine. And it’ll have come from outer space. (Geddit? Geddit? Not as lame as endless ‘Uranus’ jokes, is it…)
What’s next? Well, proof will only come with pictures. That won’t be easy. Japan’s 8.2-metre Subaru telescope, at Mauna Kea, has been tipped for the search – and can likely pick up the putative planet even if it’s at aphelion, its furthest point from the Sun. But there will be issues of ‘telescope time’ (a new planet has to be imaged at least twice, at sufficient interval to show it moving against the background stars, which is longer the further out you go; and that search also isn’t the only priority). Although Subaru’s field of view is wa-a-a-a-ay better than other large telescopes, it’s still only o.5 degrees of sky at a time, which is about the diameter of the full Moon seen from Earth. Nobody knows exactly where on its possible orbit Planet 9 might be, so there’s a relatively large area to survey. That, I expect, is why the estimate I saw yesterday put the likely search time at up to five years.
The other question is what the new world will be called, if it exists. That’s up to the IAU, the body responsible for naming them. Oceanus, maybe? That was first proposed for Neptune. Certainly they should avoid a name vulnerable to weak scatological humour, like Uranus. Or maybe they shouldn’t. It could be quite amusing.
Copyright © Matthew Wright 2016